Syringes have been commonly used for many years in medical applications to inject a fluid into a patient. In recent years, syringes have been used to inject a fluid into a patient's blood vessel in the performance of an angioplasty procedure, a relatively common procedure used to remove, or at least reduce, the amount of blockage typically caused by an accumulation of plaque that may have occurred in a blood vessel, commonly an artery.
In an angioplasty procedure, a balloon-tipped catheter or balloon is inserted into the artery involved in the procedure. Thereafter, a fluid is injected at an accurately controlled, relatively high pressure, in the order of 25-30 atmospheres, into the balloon or catheter to inflate the balloon. As the fluid fills the balloon, the latter inflates to fill the artery, thereby to compress the plaque in the artery. The physician then withdraws the fluid from the balloon causing the balloon to deflate, after which the catheter is removed from the patient.
A syringe is the preferred device used by physicians to introduce fluid into the balloon catheter. It has been found, as described in U.S. Pat No. 5,047,015, that in order to provide sufficient fluid pressure as well to achieve precise control over the fluid pressure, it is advantageous to employ a threaded plunger which can be freely, rapidly, axially reciprocated in a barrel, particularly when the balloon is being deflated. When greater control of fluid pressure, as well as a mechanical advantage to provide increased pressure, is desired, the threaded plunger engages mating threads on the interior surface of the barrel. When the plunger and barrel are thus threadably engaged, the syringe is locked, such that free, sliding, reciprocating movement of the plunger in the barrel is prevented. The plunger, however, in this condition or mode of operation can be screwed into, and thereby moved axially along the barrel in precisely controlled increments.
In the syringe described in the aforesaid U.S. Patent, transition of the syringe between the locked, threaded mode and the freely reciprocating mode of operation is achieved by the manual operation of an actuating trigger mechanism. Although this syringe is generally effective to achieve the desired high pressure and accurate control over the fluid pressure, the actuating trigger mechanism is relatively complex and is made up of a relatively large number of components, which adds to the overall cost of the syringe. There is thus a need for a syringe for use in angioplasty and related procedures that provides the precise control over the high fluid pressure required in such a procedure with a syringe that is less complex and less costly than those presently available.